Bin level indicators of the rotary paddle type, as briefly described above, are disclosed in many prior patents, including the Jett U.S. Pat. No. 4,318,624, issued Mar. 9, 1982, in which the present applicant was the applicant.
The present invention is directed to the problem of providing an improved shaft seal construction, to produce a rotary seal between the rotary shaft and the housing of such a bin level indicator of the rotary type. A seal is needed to prevent leakage around the shaft, either between the bin and the housing of the bin level indicator, or between the housing and the bin, or both.
The interior of the housing is generally at atmospheric pressure. In some cases, the interior of the bin is pressurized, above atmospheric pressure, so that the shaft seal needs to withstand the pressure in the bin, to prevent leakage between the bin and the housing. If such leakage occurs, the bin level indicator may be damaged by dust or other material, leaking from the bin. Moreover, the leakage of any material from the bin is always undesirable.
In other cases, some degree of vacuum may exist in the bin, so that the interior of the bin is at a subatmospheric pressure. In that case, a vacuum seal is needed to prevent leakage of air into the bin from the housing of the bin level indicator.
Any shaft seal tends to produce a frictional drag on the rotary shaft. The shaft seal needs to be constructed to minimize such drag, so as to prevent the drag from unduly impeding the rotation of the shaft.